1. Field of the Invention
The present invention relates to a differential amplification circuit which has a differential amplifier having two input terminals for outputting a difference between first and second signals applied to the two input terminals and more particularly, to a offset drift reducing device for a differential amplification circuit which can reduce an offset drift generated at a differential amplifier in the differential amplification circuit.
2. Description of the Related Art
An example of the application of a differential amplification circuit is that it receives a D.C. voltage supplied from a temperature sensor at its one input terminal and a D.C. voltage corresponding to a reference temperature at the other input terminal, and outputs a difference between these D.C. voltages. The differential amplification circuit comprises an operational amplifier. However, the operational amplifier usually generates an offset voltage drift. For a general operational amplifier, this offset voltage drift is 50 uV/Cmax. Assuming that the differential gain is 25 and a temperature range is between 0.degree. and 40.degree. C., a temperature drift becomes 50 mV. For the purpose of reducing such a temperature drift, a low temperature-drift operational amplifier such as OP07 manufactured by the PMI company has been often employed.
FIG. 3 shows an example of a conventional circuit which includes a buffer Q1' for inputting a signal from a sensor, a buffer Q3' for inputting a reference voltage and a differential amplifier Q2'. The buffers Q1', Q3' and differential amplifier Q2' comprise an operational amplifier OP07 as mentioned above, respectively. To the buffer Q1', a voltage V1, which is detected at the sensor such as a thermistor, is applied. Then, the buffer Q1' generates an offset voltage e1. To the buffer Q3', a reference voltage V2 corresponding to a target temperature is applied. Then, the buffer Q3' generates an offset voltage e3. The outputs of the buffers Q1' and Q3' are applied to the differential amplifier Q2', which outputs a difference voltage Vd between the outputs of the buffers Q3' and Q1'. The difference voltage Vd is expressed as follows where A is a gain of the differential amplifier Q2'. EQU Vd=A.times.(V2-V1+e2+e3-e1) (1)
The equation (1) shows that the output of the differential amplifier Q2' contains an offset voltage of A.times.(e2+e3-e1) in addition to the difference voltage of A.times.(V2-V1). In this circuit, since the low temperature-drift operational amplifier OP07 is used for the buffers Q1', Q2' and the differential amplifier Q3', the offset voltage A (e2+e3-e1) is small and thus a temperature drift is reduced.
However, it is difficult to incorporate the low temperature-drift operational amplifiers in an application specific integrated circuit (ASIC). This is because the operational amplifier OP07 is 4 to 5 times larger in size than a general operational amplifier, and the operational amplifier OP07 cannot be accommodated in a chip.
As described above, the temperature drift could be reduced by using the low temperature-drift (low offset) operational amplifiers in the circuit of FIG. 3. However, it is difficult to make such circuits in the form of ASIC.